Global industrial demand for molybdenum is high, especially with regard to metallurgical applications. Steels, cast irons, superalloys and welding alloys are important molybdenum-containing end products exhibiting enhanced strength, toughness, wear and corrosion resistance. Important non-metallurgical applications include uses as lubricants and catalysts in petroleum refining processes, paint and dye pigments, and chemical usage in flame retardants and smoke suppressants.
Molybdenite (MoS2) is the primary mineral source of molybdenum. Molybdenite containing ore can be extracted from primary mines for molybdenite. The chief ore is widely distributed, frequently occurring in small veins or scattered as small flakes, and is often associated to with granites, pegmatities or copper sulfides. Therefore, molybdenite is also frequently a by-product in copper mining. Following grinding and flotation operations, copper sulfides give rise to concentrates which are again mechanically processed to obtain molybdenite flotation concentrates. Up to 50% of molybdenite may be lost due to numerous grinding and flotation steps. The molybdenum content in these concentrates is about 45%. This low yield is particularly unsatisfying with regard to the current demand. Further, processing such concentrates by conventional pyrometallurgical technologies has an unfavorable environmental pollution impact and high energy costs.
One family of technologies that has been under development and, in some cases, commercialized, is the integration of biologically based processes to the recovery of metals from low grade ores or high grade concentrates. Two terms are used to describe distinct yet related processes: biooxidation and bioleaching. Both terms refer to the microbially-assisted degradation of sulfide-based minerals. It is a biochemical process which involves a complex interaction between microorganisms, leach solution and mineral surface. Biooxidation is typically used to describe microbially augmented oxidation of minerals such as pyrite (FeS2) and arsenopyrite (FeAsS). Typically, the goal is not to recover iron or arsenic from the sulfides, but to degrade and remove these minerals as they contain refractory precious metals such as gold locked inside. Biooxidation of pyrite and arsenopyrite in refractory gold ores has been applied on a commercial scale using both large heaps of low grade ore and in stirred reactors for concentrates. Following this biological pretreatment, gold is recovered using conventional leaching processes. Conversely, bioleaching refers to the same basic microbiological process, but with the alternative goal of recovering the solubilized metals comprising the sulfide mineral. Hence, in the special case of cobaltous pyrite, bioleaching was applied on a commercial scale to recover cobalt disseminated within the pyrite crystal matrix. Bioleaching is currently used in many places in the world on a commercial scale to recover copper from copper minerals such a chalcocite (Cu2S) and covellite (CuS). Bioleaching has also been commercially applied to ores of uranium, with processes for nickel and zinc sulfides currently at pilot scale.
Metal sulfides were once thought to be degraded by concurrent reactions which were either non-biologically mediated, such as oxidation of the sulfide by Fe(III), or by enzymatically mediated attacks on the crystal structure of the sulfide. These were collectively referred to in the microbiology literature as the “indirect” and “direct” mechanisms, respectively. Recently, features of these classical descriptions have been refined and melded (Schippers and Sand (1999) Appl. Environ. Microb. 65, 319-321) and two distinct mineral specific indirect mechanisms proposed: 1) the thiosulfate mechanism (for example, pertaining to FeS2, MoS2, and WS2) and 2) the polysulfide mechanism (for example, for ZnS, CuFeS2, and PbS). In the context of this work, the function of iron(III) hexahydrate ions is to chemically attack the acid-insoluble metal sulfides pyrite and molybdenite and to further oxidize the generated thiosulfate to sulfuric acid. Efficiency of the process is probably greatly enhanced by extracellular polymeric material produced by the cells which aids in attachment of the cells to the surface of the mineral and complexing and concentrating of Fe(III) at the mineral/cell interface. Several leaching strategies may be employed simultaneously by a mixed population.
Substantial progress has been made in identifying the various microbial populations capable of contributing to metal sulfide degradation in biooxidation or bioleaching processes. Collectively, these populations are referred to as extremophiles, as their normal environment can be characterized as a metal laden dilute sulfuric acid solution. Bacteria typifying a mesophilic temperature regime (20° C.-42° C.) include among others Acidithiobacillus ferrooxidans, A. thiooxidans, and Leptospirillum ferrooxidans. A taxonomically separate group, the Archaea, may be represented by one or more species of Ferroplasma, such as F. acidiphilum. Moderate thermophiles, for example, Acidithiobacillus caldus, Sulfobacillus acidophilus, S. thermosulfidooxidans and Acidimicrobium ferrooxidans, may attain dominance as the temperature increases further to approximately 55° C. Leaching environments attaining temperatures upwards of 65° C. or somewhat higher may be dominated by extreme thermophiles which include additional members of Archaea such as Acidianus brierleyi, Metallosphaera sedula, and Sulfolobus metallicus. 
Because metal sulfide oxidation has an electrochemical component, the solution oxidation-reduction potential, or redox potential, is important in bioleaching systems. While more precise technical arguments would include consideration of the mixed (corrosion) potential of the sulfide mineral during microbially augmented oxidation, monitoring solution redox potential is a more convenient and practical operational indicator. The redox potential is governed largely by the molar ratio of Fe(III) to Fe(II) in solution and can be expressed through the Nernst equation and be easily measured in the field or lab by a probe. A high redox potential requires that most of the iron in solution be present as Fe(III), with the primary ion actually being Fe(III) hexahydrate. In both mechanisms, the microbial populations serve to control the redox potential by cyclically oxidizing ferrous iron back to ferric iron as it is consumed by reaction with the sulfide mineral. However, not all iron-oxidizing species found in similar environments are capable of generating extremely high redox potentials since they are inhibited at high concentrations of Fe(III). For example, it is known that an iron-oxidizer such as Leptospirillum ferrooxidans can thrive at much higher potentials than Acidithiobacillus ferrooxidans. 
Some metal sulfides, including chalcopyrite (CuFeS2) and molybdenite, resist microbial bacterial attack to varying degrees and, to date, molybdenite has been considered particularly recalcitrant. First, it was observed that molybdenite leaching kinetics were unfavorable. The reported slow biooxidation rate of molybdenite suggested at least that fine particle sizes and consequent high surface areas may have been required for reasonable biooxidation rates. In addition to its crystalline structure and peculiar electronic configuration, it was noted that the solubility product for molybdenite was found to be highly predictive of its recalcitrant leaching behavior. Notwithstanding these considerations, the observed recalcitrance also appeared to result in part from limitations imposed by the requirement for a very high redox potential or, in other words, high microbial iron-oxidizing activity in the presence of toxic molybdate ions. This has been difficult to achieved during bioleaching, as concluded by Romano et al. (2001) FEMS Microbiology Letters 196, 71-75. In contrast to other problematic sulfides, such as chalcopyrite, to which tremendous efforts have been applied, there has been little additional work over the past nearly 50 years to develop approaches to bioleach molybdenite. Leaching of commercial material under naturally occurring conditions has, prior to the current invention, been considered impractical.
Tributsch and Bennett (1981) J. Chem. Technol. Biotechnol. 31, 565-577, discussed the extreme resistance of molybdenite to bacterial attack and chemical oxidation. They showed molybdenite is not attacked by protons but is attacked oxidatively by ferric ions, albeit very slowly. Molybdenite alone was not a suitable energy source for bacteria, but it slowly reduced Fe3+ added to cultures of T. ferrooxidans containing molybdenite, resulting in an increase in microbial growth via Fe2+ oxidation.
Attempts to address the issue of molybdate toxicity to ore leaching microbial populations have been reported in the literature. An adaptation study was carried out by Duncan et al. (1967) AIME Transactions 238, 122-128. The mesophilic leaching bacterium Thiobacillus ferrooxidans (now Acidithiobacillus ferrooxidans) slowly adapted over a series of six transfers with the result of growing, albeit at a slower rate, in 90 ppm molybdenum.
More recently, Nasernejad et al. (2000) Process Biochemistry 35, 437-440, used a similar strategy, in this case fifteen sequential transfers from 1 ppm ammonium molybdate to a final concentration of 15 ppm ammonium molybdate. Molybdenum sulfide was oxidized by the microorganism T. ferrooxidans in a leach solution comprised of 0.9K mineral salts solution containing 0.9 g/l Fe as ferrous sulfate. Although the final yield was about 93%0, the process involved several washing steps with hydrochloride acid and carbon disulfide, respectively, and a weekly exchange of leaching medium to reduce microbial inhibition, corresponding to a maximum concentration of about 800 mg/l Mo.
Brierley and Murr (1973) Science 179, 488-490, described the use of a thermophilic microorganism at a temperature of 60° C. for bioleaching. The organism, now known as Acidianus brierleyi, demonstrated a higher resistance to Mo compared to mesophiles, growing at a dissolved Mo concentration of up to 750 mg/l. Respiration in the absence of growth occurred up to 2000 mg/l of Mo (Brierley, 1973, J. Less Common Metals 36, 237-247). Nevertheless, molybdenum was only solubilized for a yield of 3.3% over a 30-day period. A supplement of 0.02% yeast extract and 1% ferrous sulfate increased the yield to 13.3%, but it remained undetermined whether the ferrous iron may have afforded any protective properties beyond its contribution to indirect leaching.
It has already been known from the prior disclosure of Bryner and Anderson (1957) Ind. Eng. Chem. 49, 1721-1724, that the amount of formed soluble molybdenum was increased when pyrite and molybdenite were bioleached together, thereby indicating an effect of soluble iron on the increased biological oxidation of molybdenite. However, the authors determined a definite optimum ferrous iron concentration at 4.000 ppm which yielded a total of 140 mg of soluble molybdenum concentration extracted from 5 g of molybdenite concentrate. Furthermore, it was shown that the amount of leaching was proportional to the particle size. Neither the yield nor the tolerance to molybdenum are enhanced to economic levels by considering the consistent results of the above documents.
Karavaiko et al. (1989) in Salley et al. (eds.) Proc. Int. Symp. CANMET SP 89-10, 461-473, described the saturation limit of dissolved Fe and Mo in iron containing (9K) medium during T. ferrooxidans growth and ferrous iron oxidation. Molybdenum and ferric iron occurred in both the liquid phase and in precipitates depending on their concentrations and the amount of inoculum. Sedimentation of Mo(VI) was virtually absent at pH 2.4-2.5 if its initial concentration did not exceed 250 mg/l, whereas ferric iron started to sediment in the presence of 750 mg/l Mo(VI). The solubility restrictions resulted in an effective concentration of 2443 mg/l ferric iron when a 30% inoculum was added to the culture medium, resulting in a tolerance of the organisms to 500 mg/l Mo(VI). A 20% inoculum corresponded to addition of 1675 mg/l ferric iron and 150 mg/l Mo(VI) was tolerated. Even though the authors acknowledged a contribution of ferric iron to increased T. ferrooxidans resistance due to chelating and partially sedimenting Mo(VI), the important protective role was assigned to amino acids forming composite iron-molybdenum complexes. Adaptation of T. ferrooxidans to Mo and other heavy metals was attributed to selection of mutants with increased synthesis of chelating exometabolites (amino acids). The authors suggested that a decrease in toxicity by chelation or precipitation could depend on media composition.
Use of leach solution chemistry to control toxicity of ions leached from ore has corollaries in other bioleaching applications. For example, Sundkvist, Sandström, Gunneriusson and Lindström (2005) Proc. 16th International Biohydrometallurgy Symposium, D. E. Rawlings and J. Petersen (eds.), 19-28, demonstrated that fluoride toxicity to bioleaching microorganisms could be minimized by the addition of aluminum to the leach solution.